Practically every sensor unit ever marketed for determining fill level in the case of liquids and bulk goods and for determining flow rate of a medium through a pipe system, operating based on electromechanical transducers (Vibronik), has used sinusoidal or rectangular, electrical, alternating voltage signals as drive signals for a fundamental wave excitation of the sensor units. The alternating signals are normally produced by means of an analog oscillator and analog filtered for further processing, rectified, and, in the case of limit level switches, compared by means of analog comparators with predetermined threshold values. Microprocessors are, as a rule, only used for linearizing, scaling and providing with time delays, switching hystereses, or inversions, the signals prepared by means of analog electronics.
The weakness of these sensor units shows itself in the case of accretion formation. In various media, e.g. cement, it is possible that a layer of the medium will cling to the sensor, while the fill level is still below the desired level. This layer can damp the oscillation of the sensor to such an extent that the electronics switches to the “covered” state. Furthermore, in the case of a sensor in the form of an oscillation fork, an unsymmetric accretion deposit can lead to transmission of the oscillation to the housing and the container. In this way, so much energy is withdrawn from the oscillation system, that it possibly can become completely damped. Such a behavior likewise leads to erroneous measurements.